Optical waveguides and other light-guiding or light-managing structures provide much of the terrestrial high-speed infrastructure of the telecommunications industry. Light-guiding waveguides, which are formed by surrounding a higher refractive index core with a lower refractive index cladding, support transmissions of large amounts of optical information over long distances with little signal attenuation. Light-managing waveguides include such structures as optical gratings, which are formed by index perturbations spaced along limited lengths of the waveguides to separate narrow bands of wavelengths from broader band signals.
The waveguides generally take fiber or planar forms fashioned from doped silica-based materials exhibiting contrasting refractive indices. Further variations in refractive indices for forming gratings and other optical structures can be made by exposing photosensitive optical materials to patterns of actinic radiation within the absorption spectra of the optical materials. The absorption mechanism limits the photo-induced variations to surfaces or regions near the surfaces of homogeneous optical materials.
Recently, high energy pulses beyond the absorption edge of silica-based materials have been demonstrated to produce refractive index changes inside bulk glass. Such changes open possibilities for manufacturing three-dimensional optical circuitry in which light-guiding or light-modifying structures are formed throughout glass volumes. Eventually, this capability is expected to simplify manufacture and reduce space requirements of optical structures performing complex or numerous optical functions.
For example, an 810 nanometer (nm) wavelength Ti:Sapphire laser emitting 120 femtosecond (fs) pulses at 200 kilohertz (kHz) has been used to direct write the cores of waveguides in silica-based glass samples. The pulses at 810 nm are well beyond the absorption edge of the silica glass samples. Focused laser beam power reaching the glass samples was regulated by filters between 40 and 800 milliwatts (mW). Translation speeds between the laser beam and the glass samples varied between 100-10,000 microns per second (μm/s). Refractive index (n) increases of nearly 0.04 were reported, which apparently resulted from repeated exposures. Core diameters written into glass samples varied as a function of the average beam power reaching the samples.
The mechanism responsible for the index change in the silica-based glass samples is not well understood. However, since the index change is produced by high energy pulses at wavelengths beyond the absorption edge of the glass samples, multiphoton (i.e., non-linear) absorption is believed to be at least partially responsible. Speculations relating to the changes in the glass include local densification, the formation of color centers, lattice defects, and melting. Reports suggest that increasing the peak power or the duration of exposure increases the change in refractive index.
Amplified femtosecond pulse sources, such as the Ti:Sapphire laser referred to above, have pulse rates in the kilohertz (kHz) range and pulse energies in the microjoule (μJ) range. Since the thermal diffusion time of silica and related materials is in the order of a few microseconds (μs), each pulse heats independently of the others. However, the amplified femtosecond pulses have sufficient energy to raise the instantaneous temperature of the glass materials to 1000 degrees centigrade (°C.) or more, which is large enough to produce local thermal damage.
While the requisite index changes for writing waveguides have been demonstrated, actual light-guiding properties associated with the index changes in bulk glass materials have been inconsistent. At least some of the material changes including the formation of voids and other defects associated with the change in index physically damage the index-modified glass. The physical damage can attenuate optical signals transmitted through the glass.
To make the femtosecond laser direct-write method practical, substantial changes in the refractive index (e.g., >10−3) of a material must be achieved in a reasonable amount of writing time without incurring physical damage that interferes with the intended waveguiding function. Such a method could be used to write continuous light-guiding waveguide patterns connecting any two points within a continuous block of a suitable material or make other optical devices, such as optical gratings.